Thanks for the thoughtful comments and recommendations. We have contacted Watkins, who advised us to follow the manual instructions, which says to use ph/alkalinity down, although it also says muriatic acid may be used as substitute. I appreciate any input from anyone who can find out more about it. We enjoy our Hot Spring Vista very much. It seems to me, however, that there are quite a few more bugs to work out in the ACE salt water system and we probably would not have purchased it yet had we known how much trouble it would be (the salt water system, NOT the hot tub!).